impenetrable (adj.)youdaoicibaDictYouDict[impenetrable 词源字典]
mid-15c., from Middle French impenetrable, from Latin impenetrabilis "that cannot be penetrated," from assimilated form of in- "not, opposite of" (see in- (1)) + penetrabilis "penetrable" (see penetrate). Related: Impenetrably; impenetrability.[impenetrable etymology, impenetrable origin, 英语词源]
penetrable (adj.)youdaoicibaDictYouDict
early 15c., "penetrating," from Latin penetrabilis "penetrable, vulnerable," from penetrare (see penetrate). Meaning "capable of being penetrated" is attested from 1530s; figurative use by 1590s. Related: Penetrability.